The general structures and manufacturing processes for electronic packages are described in, for example, Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y., (1988), and Rao R. Tummala and Eugene J. Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), both of which are hereby incorporated herein by reference.
As described by Seraphim et al., and Tummala et al., an electronic circuit contains many individual electronic circuit components, e.g., thousands or even millions of individual resistors, capacitors, inductors, diodes, and transistors. These individual circuit components are interconnected to form the circuits, and the individual circuits are interconnected to form functional units. Power and signal distribution are done through these interconnections. The individual functional units require mechanical support and structural protection. The electrical circuits require electrical energy to function, and the removal of thermal energy to remain functional. Microelectronic packages, such as, chips, modules, circuit cards, circuit boards, and combinations thereof, are used to protect, house, cool, and interconnect circuit components and circuits.
Within a single integrated circuit, circuit component to circuit component and circuit to circuit interconnection, heat dissipation, and mechanical protection are provided by an integrated circuit chip. This chip enclosed within its module is referred to as the first level of packaging.
There is at least one further level of packaging. The second level of packaging is the circuit card. A circuit card performs at least four functions. First, the circuit card is employed because the total required circuit or bit count to perform a desired function exceeds the bit count of the first level package, i.e., the chip. Second, the second level package, i.e., the circuit card, provides a site for components that are not readily integrated into the first level package, i.e., the chip or module. These components include, e.g., capacitors, precision resistors, inductors, electromechanical switches, optical couplers, and the like. Third, the circuit card provides for signal interconnection with other circuit elements. Fourth, the second level package provides for thermal management, i.e., heat dissipation.
In order for the card to accomplish these functions the I/C chip must be bonded to the card, and connected to the wiring of the card. When the number of I/O's per chip was low, serial wire bonding of the I/O's around the periphery of the chip was a satisfactory interconnection technology. But, as the number of I/O's per chip has increased, tape automated bonding (hereinafter "TAB" bonding) has supplanted serial wire bonding. To handle an even larger number of I/O's per chip various "flip chip" bonding methods were developed. In these so-called "flip chip" bonding methods the face of the IC chip is bonded to the card.
Flip chip bonding is described by Charles G. Woychik and Richard C. Senger, "Joining Materials and Processes in Electronic Packaging," in Donald P. Seraphim, Ronald Lasky, and Che-Yo Li, Principles of Electronic Packaging, McGraw-Hill Book Company, New York, N.Y., (1988), at pages 577 to 619, and especially pages 583 to 598, and by Nicholas G. Koopman, Timothy C. Reiley, and Paul A. Totta, "Chip-To-Package Interconnections" in Rao R. Tummala and Eugene Rymaszewski, Microelectronic Packaging Handbook, Van Nostrand Reinhold, New York, N.Y. (1988), at pages 361 to 453, and especially pages 361 to 391, both of which are hereby incorporated herein by reference. As described therein. flip-chip bonding allows forming of a pattern of solder bumps on the entire face of the chip. In this way the use of a flip chip package allows full population area arrays of I/O. In the flip chip process solder bumps are deposited on solder wettable terminals on the chip and a matching footprint of solder wettable terminals are provided on the card. The chip is then turned upside down, hence the name "flip chip," the solder bumps on the chip are aligned with the footprints on the substrate, and the chip to card joints are all made simultaneously by the reflow of the solder bumps.
In the C4 process, as distinguished from the earlier flip chip process, the solder wettable terminals on the chip are surrounded by ball limiting metallurgy ("BLM"), and the matching footprint of solder wettable terminals on the card are surrounded by glass dams or stop-offs, which are referred to as top surface metallurgy ("TSM"). These structures act to limit the flow of molten solder during reflow.
The ball limiting metallurgy ("BLM") on the chip is typically a circular pad of evaporated, thin films of Cr, Cu, and/or Au, as described, for example by P. A. Torta and R. P. Sopher, "STL Device Metallurgy and Its Monolithic Extension," IBM Journal of Res. and Dev., 13 (3), p. 226 (1969), incorporated herein by reference. The Cr dam formed by this conductive thin film well restrains the flow of the solder along the chip, seals the chip module, and acts as a conductive contact for the solder. In prior art processes the BLM and solder are deposited by evaporation through a mask, forming an array of I/0 pads on the wafer surface. The term "mask" is used generically. The mask can be a metal mask. Alternatively, as used herein, the "mask" can refer to a sequence of BLM deposition, photoresist application, development of the photoresist, and deposition, as described below, of solder, followed by simultaneous removal of the photoresist and subetching of the BLM, with the solder column acting as a mask.
In C4 processes the Pb/Sn is typically deposited from a molten alloy of Pb and Sn. The Pb has a higher vapor pressure then Sn, and deposits first, followed by a cap of Sn. The solder is deposited on the chip by evaporation, vacuum deposition, vapor deposition, or electrodeposition into the above described BLM wells, thereby forming solder columns therein. The resulting solder deposit, referred to herein as a column or a ball, is a cone-frustrum body of Pb surrounded by an Sn cap. This column or ball may be reflowed, for example by heating in an H.sub.2 atmosphere, to homogenize the solder and form solder bumps for subsequent bonding.
The solder is typically a high lead solder, such as 95 Pb/5 Sn. In conventional C4 processes, 95/5 solders are preferred because the high lead solders of this stoichiometry have a high melting point, e.g., above about 315 degrees Centigrade. Their high melting temperature allows lower melting point solders to be used for subsequent connections in the microelectronic package.
The wettable surface contacts on the card are the "footprint" mirror images of the solder balls on the chip I/O's. The footprints are both electrically conductive and solder wettable. The solder wettable surface contacts forming the footprints are formed by either thick film or thin film technology. Solder flow is restricted by the formation of dams around the contacts.
The chip is aligned, for example self-aligned, with the card, and then joined to the card by thermal reflow. Typically, a flux is used in prior art C4 processes. The flux is placed on the substrate, or chip, or both, to hold the chip in place. The assembly of chip and card is then subject to thermal reflow in order to join the chip to the card. After joining the chip and card it is necessary to remove the flux residues. This requires the use of organic solvents, such as aromatic solvents and halogenated hydrocarbon solvents, with their concomitant environmental concerns.
To be noted is that the C4 process is a substantially self-aligning assembly process. This is because of the interaction of the geometry of the solder columns or balls prior to reflow with the surface tension of the molten solder during reflow and geometry of the solder columns. When mating surfaces of solder column on the chip and the conductive footprint contact on the card touch, the surface tension of the molten solder will result in self alignment.
C4 bonding can also be used in Direct Chip Attach ("DCA"). Direct Chip Attach is the general name applied to a chip-substrate connection used to eliminate the first level of packaging (as SMT or TAB). Direct Chip Attach is a low cost, high throughput process, in which the chip is bonded, for example, C4 bonded, directly to the card or board.
For Direct Chip Attach, the individual IC chips are (C4) mounted on the cards or boards, and the mounted chip and the card or board are then encapsulated with a low coefficient of thermal expansion encapsulant, as epoxy resin. By this expedient, the "standoff" between the IC chip and the card or board is encapsulated with epoxy.
Because of the use of a polymeric dielectric card or board, the Direct Chip Attach process requires low temperature solder metallurgy that also allows "stand-off." Moreover, Direct Chip Attach encapsulation increases the resistance of the C4 solder interconnections to thermal cycling, acts as an alpha emission barrier to MOSFET memory chips, is a parallel thermal path for heat dissipation, and provides physical protection to the chips and C4 solder interconnections.
One problem encountered with the combination of Direct Chip Attach and and C4 bonding is the difficult of reworking the encapsulated package. This is because, notwithstanding the advantages of the epoxy encapsulant, for example: (i) low temperature encapsulant cure, (ii) easy manufacturability, (iii) the absence of volatiles, and (iv) thermal expansion matched to that of the C4 joints, epoxy is a difficult material to remove for rework.
This rework difficulty renders the structures effectively non-reworkable. This reduces yield and thereby increases costs.